Privacy-enhanced e-passport authentication protocol

ABSTRACT

A passport authentication protocol provides for encryption of sensitive data such as biometric data and transfer of the encryption key from the passport to the authentication authority to permit comparison to a reference value.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/436,986 filed on May 19, 2006 and claims priority from U.S.Provisional Patent Application No. 60/682,862 filed on May 20, 2005hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to protocols for restricting access tosensitive information embedded in documents such as passports andidentity cards.

BACKGROUND OF THE INVENTION

Existing passport security technology links identity of an individual byembedding a photograph within the passport.

The existing linkage is not cryptographically strong as substituting adifferent photograph is relatively easy. Also, the photograph iscompared manually to the face of the traveller by the border controlinspector, which has certain problems.

To enhance security, it has been proposed to provide machine-readablepassport or identity card in which biometric data is stored in a chipwithin the document and can be retrieved for examination. Typically, thebiometric data will be an iris scan, fingerprint or images of the faceof the bearer.

The International Civil Aviation Organisation (ICAO) has proposedmachine readable travel documents (MRTD), i.e. e-Passport system thatauthenticates the identity of individuals to border control stations bycryptographically linking the identity of the individual (such as nameand nationality) to biometric data for the individual.

The cryptographic linkage is obtained by digitally signing the identitydata and biometric data of the individual. The resulting signed identityand biometric information is conveyed from the passport to a passportreader. The signature binds the identity of the individual to thebiometric identity, which makes faking a passport a cryptographicallyhard problem. A concern arises however that each individual's biometricinformation is highly sensitive and should not be inadvertently madeavailable.

It is therefore an object of the present invention to obviate ormitigate the above disadvantages by making it more difficult forunauthorized parties to obtain the biometric information and othersensitive information from a document such as a passport.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described with reference tothe appended drawings wherein:

FIG. 1 is a schematic representation of a passport examination station;

FIG. 2 is a schematic representation of the components of the passportand reader;

FIG. 3 is a representation of an exchange of data within the station.

DETAILED DESCRIPTION OF THE INVENTION

Referring therefore to FIG. 1, a passport 10 includes a chip 12 and aradio frequency identification (RFID) tag 14 with an antenna 16. Areader 20 includes an antenna 22 to communicate with the antenna 16 anda scanner 24 to obtain a reference input from the 18, bearer of thepassport 10. The reference input may be a real time fingerprint scan oriris scan or a facial image. The reader 20 includes a data processingengine 26 to manipulate data received from the passport 10 and scanner24 and a screen 28 to view the results of such manipulation. An inputdevice 30, such as a keyboard or mouse is included to permit userinputs.

As shown in FIG. 2, the chip12 contains a memory 32 to store biometricdata and personal information such as name, nationality and date ofbirth. The memory 32 is designed to be tamperproof and communicates witha cryptographic unit 34 and data transmission network 36 connected tothe antenna tag 14.

The cryptographic unit 34 includes an arithmetic processor 38 forperforming cryptographic operations and a secure memory 40 for storingprivate keys and certificates. Preferably, the underlying cryptographicsystem is an elliptic curve cryptosystem. The cryptographic unit 34includes the parameters of the underlying system, such as the curve, andthe generator G of the points on the curve and has access to the publickey Q of the passport.

In the preferred embodiment, the memory 40 includes a private signingkey d, the corresponding public key Q=dG, and a certificate, Cert Q,which is issued by a certification authority, such as the passportissuer, which certifies the public key Q. The processor 38 can performcryptographic operations such as point addition, key derivation and hashfunctions. The cryptographic unit 34 also includes a random numbergenerator (RNG) 42 to provide integers for use as private session keys.

The data processing engine 26 of the reader 20 also includes acryptographic unit 50 including a random number generator 52 and anarithmetic processor 54.

In operation, the scanner 20 initiates a message transfer by activatingthe chip 12 through the RFID tag 14. A message M is assembled consistingof the data required for processing the passport and confirming identitysuch as the biometric data, bearer's name, nationality and date of birthtogether with the certificate of the bearer's public key Cert Q. Thedata utilized will depend on the information required by the passportcontrol.

The message M is divided into two, parts, M₁, M₂, with the sensitiveinformation to be maintained confidential such as the biometric datawithin the message part M₁. Less sensitive or publicly availableinformation such as the country of issue or visa is included in themessage part M₂.

A random number k is generated by the RNG 42 and a value R=kQ computed.The value R is used in a key derivation function (KDF) performed in theprocessor 38 to obtain a session encryption key e. Any suitable KDF maybe utilized, typically one utilizing a secure hash function.

The message part M₁, is checked for a predetermined level of redundancyand, if that is not met, additional data added. The session encryptionkey e, is used to encrypt the message part M₁ to cyphertext C. Thecyphertext C is then concatenated with the message part M₂ and hashedusing a secure hash function H to obtain a value, h, i.e. h=H(C,M₂).

A signature component s is then computed using the relationship s=k+dhmod n where n is the order of the generator G.

Data is then transferred through the RF ID tag 14 including thesignature component s, the public part of the message M₂, (whichincludes the certificate of the public key Q) and the cyphertext C.

The reader 20 captures the data and initially verifies the public key Qfrom the certificate. It then computes a value V=sG-hQ and generates aprivate session key b from the RNG 52. A public session key U=bV is thencomputed and sent to the chip 12 through the RF ID connection. The chip12 confirms that the point U is a point on the curve and generates afurther public key W=dU that is sent back to the reader 20.

The reader then uses the private session key b to compute a value equalto R, namely (b⁻¹ mod n) W and then uses the KDF to get the valuecorresponding to e. Using the computed value of e, the cyphertext C isdecrypted and the biometric data in the message part M₂ is recovered.The redundancy of the recovered data is checked and, if above therequired level it is accepted.

The recovered data is then compared the reference data obtained from thescanner to authenticate the bearer of the passport.

By separating the message and encrypting the biometric data, itsconfidentiality may be maintained even to an eavesdropper.

The signing process above is quite efficient for the signer. Thecomputation of R=kQ can be done in advance, or with assistance of fixedpre-computed multiples of Q. The most expensive step for the signer iscomputing W=dU.

The data exchange may also be enhanced by providing for authenticationof the reader 20. In this way, the signer can choose whether or not tointeract with the verifier. Ideally, the verifier should authenticateitself to the signer, such as by a digital signature or some symmetrickey system. In this way, the signer can control to whom the messageportion M₁ is revealed. This can be done prior to the initial exchangeof data or during the exchange before the value W is transferred.

If the signing is too expensive computationally, then the followingmodification is possible. The verifier sets b=1. Then W=R, which thesigner has already computed during signature generation. To keep M₁confidential, this alternate approach requires that R can be sent to theverifier confidentially. In particular, passive eavesdroppers should botbe able to intercept R. This might be accomplished by physical means,such as weak RF signals, or by some form of encryption, such as thee-passport basic access control encryption system.

By utilizing the bearer's public key Q in the computation of R, thesignature cannot be verified without involvement of the bearer. Inparticular, the cyphertext C cannot be decrypted without theacquiescence of the bearer.

It will be noted that once the verifier recovers R, it can compute dQ,which can be seen to enable message recovery from the signature, thatis, without the interactive verification process.

1. A method of maintaining confidentiality of sensitive informationstored in a machine readable document pertaining to a correspondentduring transmission of said sensitive information to a machine forexamination, comprising: generating an encryption key e from a publickey of said correspondent and encrypting said sensitive information withsaid encryption key e to obtain a ciphertext C, forwarding saidciphertext C to said machine, receiving from said machine an ephemeralpublic key obtained from an ephemeral private key b of said machine andsaid ciphertext C, and returning to said machine additional informationto permit recovery of said sensitive information by said machine fromsaid ciphertext C.
 2. A method according to claim 1 further comprising:prior to forwarding one of said ciphertext C and said additionalinformation to said machine, authenticating said machine to said machinereadable document, wherein said one of said ciphertext C and saidadditional information is forwarded to said machine only upon successfulauthentication of said machine.
 3. A method according to claim 1 whereinsaid sensitive information is biometric information.
 4. A methodaccording to claim 1 wherein said ephemeral public key is used togenerate said additional information.
 5. A method according to claim 1wherein said additional information permits recovery of said encryptionkey e.
 6. A method according to claim 1 wherein said encryption key e isderived from a value R obtained from a long term public key Q of saidcorrespondent and a session private key k generated by said machinereadable document.
 7. A method according to claim 6 wherein saidadditional information permits computation of said value R by saidmachine and thereby derivation of said encryption key e.
 8. A methodaccording to claim 6 wherein said long term public key Q has acorresponding long term private key d and said additional information isobtained from combining said long term private key d and said ephemeralpublic key.
 9. A method according to claim 6 wherein said ephemeralpublic key incorporates a signature component s that binds a long termprivate key d of said correspondent and said session private key k witha hash h of said ciphertext C.
 10. A method of maintainingconfidentiality of sensitive information stored in a machine readabledocument pertaining to a correspondent during transmission of saidsensitive information to a machine for examination, comprising: saidmachine initiating a request to assemble a message by said machinereadable document, said message having a primary portion M₁ and asecondary portion M₂, said primary portion including a ciphertext Cobtained from encrypting said sensitive information with a sessionencryption key e and said secondary portion containing less sensitiveinformation retrieved from said machine readable document and includinga long term public key of said one correspondent, receiving said messagefrom said machine readable document, generating a value from saidciphertext C and said secondary portion M₂, generating an ephemeralprivate key, and utilizing said value and said ephemeral private key togenerate a public session key from said value and said ephemeral privatekey, forwarding said public session key to said machine readabledocument and obtaining from said machine readable document additionalinformation to permit recovery of said sensitive information from saidciphertext C, and said machine recovering said sensitive informationfrom said ciphertext C using said additional information.
 11. A methodaccording to claim 10, wherein said additional information is a furtherpublic key that permit recovery of said session encryption key.
 12. Amethod according to claim 10, further comprising: said machine comparingsaid recovered sensitive information with a reference input obtaineddirectly from said correspondent to authenticate identity of thecorrespondent.
 13. A machine readable document having a cryptographicunit including an arithmetic processor for performing cryptographicoperations and a random number generator to provide ephemeral sessionkeys, a data communication interface for communicating with a machinefor examining said machine readable document, and a memory device tostore sensitive information in a secure manner, said data communicationinterface and said memory device being in data communication with saidcryptographic unit, said cryptographic unit performing operations toimplement the method of claim
 1. 14. A machine readable documentaccording to claim 13, wherein said cryptographic unit includes a securememory device for storing private keys and certificates.
 15. A machinereadable document having a cryptographic unit including an arithmeticprocessor for performing cryptographic operations and a random numbergenerator to provide ephemeral session keys, a data communicationinterface for communicating with a machine for examining said machinereadable document, and a memory device to store sensitive information ina secure manner, said data communication interface and said memorydevice being in data communication with said cryptographic unit, saidcryptographic unit performing operations to implement the method ofclaim
 1. 16. A machine readable document according to claim 15, whereinsaid cryptographic unit includes a secure memory device for storingprivate keys and certificates.
 17. A machine for authenticating acorrespondent based on sensitive information stored in a machinereadable document pertaining to the correspondent, comprising: a dataprocessing engine, a data communication interface coupled to the dataprocessing engine, the data communication interface being configured tocommunicate with a machine readable document having stored thereinsensitive information pertaining to the correspondent, and a scannercoupled to the data processing engine for obtaining a reference inputdirectly from the correspondent, wherein the data processing engine isconfigured to implement the method of claim 10.